You don't have to uncritically accept the Gazan Health Ministry's figures, but it is quite clear there has been significant civilian harm in Gaza in the last few weeks. The ministry did itself no favors with the hospital blast in my opinion, but even that was clearly a mass casualty event.
People like Justin Amash aren't lying about their relatives dying. We can see the dead bodies of children.
We won't get official numbers for some time. The U.N. Office for the Coordination of Humanitarian Affairs does eventually cross-check the Gazan Health Ministry's numbers with a secondary source. This work takes time though and for this current conflict it is unavailable. So any UN numbers you see are just repeating the ministry's numbers. For previous conflicts, OCHA's numbers are cross-checked.